1. Field of the Invention
The present invention relates to a method for manufacturing a hollow laminated piezoelectric actuator having a cavity such as a cylindrical through hole or U-shaped groove, and more particularly to a method of selective formation of an insulator on a surface of the cavity.
2. Description of the Prior Art
When the cavity is formed so as to extend in a direction crossing a plurality of piezoelectric ceramic layers and internal electrode layers, end faces of the internal electrodes are exposed to the air on the cavity wall surface. If the piezoelectric actuator in such a condition is driven by applying a direct current or a voltage approximating a direct current, then the metal constituting the internal electrodes is ionized and a bridge is formed between adjacent internal electrodes due to the so-called migration phenomenon. This phenomenon which is apt to take place especially when the piezoelectric actuator is driven under conditions of high humidity results in a serious problem as to the reliability because of the generation of an electrically short-circuited condition between the adjacent internal electrodes.
As a means for resolving the above problem, Japanese Laid-Open Patent Application No. 61-15382 proposed to form a uniform insulating film 14 consisting of silica (SiO.sub.2) on the wall surface of the cavity 5 by using a low pressure CVD method as shown in FIG. 1. With this arrangement, it is possible to cover all of the end faces of the internal electrodes 2 exposed on the wall surface of the cavity 5 with the silica. Accordingly, ionization of the metal constituting the internal electrodes can be construed and the migration phenomenon can be prevented.
However, the method disclosed in the above-mentioned Application includes the following two serious problems of cost and reliability.
According to the above prior art, the silica produced by bringing SiH.sub.4 and N.sub.2 O into reaction at 500.degree. C. in the low pressure CVD method is deposited to a thickness of 50 .mu.m on the wall surface of the cylindrical cavity. This thickness becomes also a significant factor that hinders productivity.
As for the reliability problem, when a voltage is applied to the piezoelectric actuator in this condition, the piezoelectric actuator expands in its thickness direction in proportion to the applied voltage, and exerts a tensile stress on the silica film deposited uniformly on the wall surface of the cavity. Therefore, when the voltage is high, namely, when the expansion of the piezoelectric actuator is large, or when the voltage is turned on and off intermittently, the silica film tends to generate cracks due to application of a large tensile stress or a cyclic tensile stress. If cracks are generated in the silica film, the end faces of the internal electrodes on the wall surface of the cavity are exposed again, and the original purpose of enhancing the reliability is defeated.